The present invention relates to generally an exhaust gas recirculation system for an internal combustion engine and more particularly an exhaust gas recirculation system of the type having an exhaust gas control valve inserted in an exhaust gas recirculation passageway communicating an exhaust system with an intake system of the engine in order to control the flow rate of the exhaust gases to be recirculated in response to a negative pressure signal, a constant pressure chamber defined in the recirculation passageway upstream of the exhaust gas control valve, and a modulator valve with a diaphragm chamber in communication with the constant pressure chamber for controlling said negative pressure signal in response to the pressure variation in the diaphragm chamber, whereby the ratio of the amount of recirculated exhaust gases to the amount of intake air may be always maintained substantially constant.
In the exhaust gas recirculation system of the type described, due to the combination of the pulsation of exhaust gases, the columnar vibrations in the exhaust system and the pressure variation caused when the exhaust gas control valve is opened, the pressure in the constant pressure chamber varies with a complicated wave form including irregular components. It is not preferable to directly transmit the pressure variation in such complex waveform to the diaphragm chamber of the modulator valve. Therefore an orifice is inserted in a passageway intercommunicating the constant pressure chamber and the diaphragm chamber of the modulator valve so that the pressure variation with a substantially sinusoidal waveform may be transmitted to the diaphragm chamber by the elimination of the irregular components of the pressure variation. However with the conventional orifice, both the positive and negative pressures of the pulsating pressure are damped. Especially the damping of the positive pressure results in the decrease in the negative pressure transmission time during which the modulator valve controls to transmit the negative pressure signal to the exhaust gas flow control valve. As a result, the time interval during which the exhaust gas flow control valve is opened is decreased so that the amount of recirculated exhaust gases is decreased accordingly, whereby the NOx emission cannot be satisfactorily reduced.